1. Field of the Invention
The present invention relates to an exposure apparatus used when semiconductors, liquid crystal display elements and the like are manufactured, and more particularly to that preferably applicable to a project exposure apparatus in which the surface of a photosensitive substrate is aligned with the image plane of a projection optical system by means of an AF sensor of an obliquely incident type to perform exposure.
2. Related Background Art
In manufacturing semiconductor elements or the like, there have been used projection exposure apparatuses such as steppers wherein a miniature pattern image of a reticle as a mask is projected and transferred to each of shot areas on a wafer (or a glass plate or the like) on which a photoresist is coated. The conventional projection exposure apparatus includes a wafer stage for sequential and rapid positioning of the shot areas on a wafer to an exposure filed, which wafer is stably held on the wafer stage.
FIG. 4 shows the upper construction of the conventional wafer stage. In FIG. 4, a wafer 1 has a photoresist coated thereon. A circular wafer holder 2 includes on the surface thereof straight line-like convex portions 3A-3E which are parallel to each other. The wafer 1 is held on the convex portions 3A-JE of the wafer holder 2 by vacuum suction. The wafer holder 2 is fixedly mounted on the upper surface of a Z-stage 4. The Z-stage 4 is rested on a Y-stage 5 for slidable movement of the Z-stage 4 relative to the Y-stage 5 along a guide (not shown). The convex portions 3A-3E provided on the upper surface of the wafer holder 2 are formed with vents for serving vacuum suction.
The Z-stage 4 and the Y-stage 5 include inclined contact portions which come into contact with each other. Sliding of the Z-stage 4 relative to the Y-stage 5 in an X-direction permits the height of the upper surface of the Z-stage 4 (or the position of the Z-stage 4 in a Z-direction) to be changed. The Y-stage 5 is movably rested on an X-stage (not shown) for movement of the Y-stage 5 relative to the X-stage in a Y-direction. The movement of the X-stage and the Y-stage 5 allows the Z-stage 4 to be positioned in the directions of X and Y. The Z-stage includes therein a .theta. table for rotating the wafer holder 2, a leveling table for adjusting the oblique angle of the wafer 1, and so on. The .theta. table and the leveling table are not shown in FIG. 4. The wafer stage comprises the Z-stage 4, the Y-stage 5, the X-stage, and so on.
A reference mark member 8 is fixedly mounted on the upper surface of the Z-stage 4 closely to the wafer holder 2. A shift mirror 10X for an X-axis and a shift mirror 10Y for a Y-axis are fixedly mounted on the upper surface of the Z-stage 4 outside of the reference mark member 8. A laser beam for measurement is supplied from a laser interferometer 11X which is provided outside of the wafer stage to the shift mirror 10X. Two laser beams are supplied from a laser interferometer 11Y to the shift mirror 10Y. Two-dimensional coordinates (X, Y) of the Z-stage 4 are determined by a value measured by the laser interferometer 11X and an average of two values measured by the laser interferometer 11Y. A rotation angle of the Z-stage 4 is obtained from a difference between the two values measured by the laser interferometer 11Y. The reference mark member 8 includes on the surface thereof a shading film which has provided therein a reference mark 9Y comprising a slit-shaped opening extending in the X-direction and a reference mark 9X comprising a slit-shaped opening extending in the Y-direction. The illumination of the reference marks 9X and 9Y from the bottom side thereof allows alignment of a reticle (not shown) to be performed on the basis of the reference marks 9X and 9Y. Each of the reference marks 9X and 9Y is referred to as "a light-emitting mark".
In FIG. 4, a predetermined shot area of the wafer area is positioned within an exposure field 6 of a projection optical system (not shown). A slit image 7 from an illumination optical system of an AF sensor (not shown) of an obliquely incident type (a focus position detecting system) is obliquely projected onto a measurement point at the center of the exposure field 6. A reflection light from the measurement point is re-focused within a light-receiving optical system of the AF sensor of an obliquely incident type. The position of the measurement point in the Z-direction (or a focus position) is determined from a value of lateral deviation of the re-focused position. Movement of the Z-stage 4 in the Z-direction to align the focus position with the position of the image plane allows auto-focusing to be effected.
In the conventional projection exposure apparatus, the wafer holder 2, the reference mark member 8, and the shift mirrors 10X and 10Y are fixedly mounted on the Z-stage 4 at the upper side of the wafer stage to protrude therefrom. The projection exposure apparatus is located within a chamber in which clean air the temperature of which is normally adjusted to be constant is circulated. Thus the upward protrusion of the above elements from the wafer stage causes air flow turbulence, thereby adversely affecting the focus quality of the projection optical system.
Similarly, in the AF sensor of an obliquely incident type in which a slit image or the like is obliquely projected within the exposure field 6 of the projection optical system, fluctuation of the wavefront of focus-detecting luminous flux caused by air flow turbulence above the wafer stage results in a slit image re-focused within the light-receiving optical system being unclear, thereby reducing accuracy when detecting a focus position.